Method of recycling adhesive-coated plastic sheet material

ABSTRACT

A method is indicated of recycling plastic sheet material coated with adhesive and available in chopped form. The method provides for separating the plastic from the adhesive. In a first method step the chopped sheet material (4) is introduced into a solvent bath (1) where the chopped sheet material (4) is moved for a predetermined dispersing time under mechanical action. The mechanical action preferably is exercized by rolling or kneading. In a second method step, the adhesive dispersed in the solvent is separated from the chopped sheet material (4) by mechanical action of forces--preferably by introducing squeezing forces in a conveyor screw.

FIELD OF THE INVENTION

The instant invention relates to a method of recycling plastic sheetmaterial coated with adhesive and available in chopped form byseparating the plastic from the adhesive.

BACKGROUND, OBJECTS AND SUMMARY OF THE INVENTION

The underlying problem in the first place pertains to the art ofmanufacturing adhesive tapes from plastics but it is transferableequally well to any kind of plastic material which is coated withadhesive. The manufacture of plastic adhesive tapes and of the othermaterials mentioned involves the production of waste, for instance, inthe form of marginal strips cut off from the material in themanufacturing process. In a more general sense the term "plastic wastecoated with adhesive" preferably is understood as relating to waste ofthermoplastic materials, especially polyethylene, polypropylene,polystyrene, polyester, and polyvinyl chloride (PVC). Even in optimizedmanufacturing processes the proportion of this waste--among others alsodue to rejects--still accounts for some 10% of the production and, as arule, results in the form of chopped sheet material of varying size,preferably between 1 and 10 mm long, at the manufacturer's of plasticfilm or sheet material. It is not only economic for reasons but alsounder environmental aspects that renewed use is aimed at of thisadhesive coated plastic sheet material waste in the form of itsseparated components.

An economically operating process of the kind mentioned initially isunknown. True, DE-OS 38 31 023 discloses a beneficiation method forreuse of soiled plastic products wherein the chopped sheet material isagitated in a washing solution of organic solvents and subsequentlyremoved from the washing solution in a turbulence-free zone, thenpredried mechanically by centrifugal force, and thereafter redriedthermally by hot gas, the solvent being recycled from the drying stageinto the washing stage. However, this known method is not suited toachieve the aim of the instant invention, namely the sustainedseparation of the adhesive from the plastic sheet material, or not so inan economic manner. Rather, the known method is directed at cleaning theplastic waste mentioned from contamination; the method steps in thatcase are not adapted to bring about sufficient separation of adhesivesfrom the chopped sheet material, which adhesives stick quite firmly tothe chopped sheet material, such as rubber-based adhesives which are notsoluble in water and glycol.

It is the object of the invention to indicate a method of the kindmentioned initially for separating the adhesive from the chopped sheetmaterial by means of a solvent in an economic and reliable way.

This object is met, in accordance with the invention, in that

(a) the chopped sheet material is kneaded for a predetermined period oftime while the solvent is being added so that the adhesive will becomedispersed in the solvent;

(b) subsequently the solvent-adhesive dispersion is separated from thechopped sheet material by the action of mechanical force and discharged.

The kneading of the adhesive-coated chopped sheet material in thepresence of the solvent in method step (a) turns the adhesive and thesolvent into a pasty, honey-like dispersion which can be separated fromthe chopped sheet material in the subsequent method step (b), forexample by squeezing or shearing forces. It is an advantage of the novelprocess that reliable separation is achieved and that it is economicalbecause the residence time of the chopped material is short and littlesolvent is used as compared to the amount of adhesive to be separated.In addition, the adhesive is yielded in the form of a solvent-adhesivedispersion which also permits direct renewed used of the adhesive, suchas by applying it on sheet material.

To obtain particularly clean chopped sheet material, the novel methodpreferably is carried out in two stages, or best in three stages, wheremethod steps (a) and (b) present one first stage of the process which isfollowed by a second stage or by second and third stages, eachcomprising steps (a) and (b). Here the chopped sheet material beingtreated always comes from the preceding stage, the solvent-adhesivedispersion obtained in the second stage and, where applicable, in thethird stage is supplied to the respective preceding stage as thesolvent, and fresh solvent is used only in the last stage.

To carry out method step (a), it is preferably provided that thedispersing of the adhesive sticking to the chopped sheet material iseffected with the solvent in a respective closed kneader, especially amulti-stage kneader. It may be sufficient to use a kneader with lesskneading steps in the second or third stages of the method than in thefirst ones. Suitable kneaders are commercially available.

As regards the second method step (b), it is preferably suggested thatthe separation of the dispersed adhesive from the chopped sheet materialbe effected by introducing squeezing forces in a conveyor screw whosevolume per unit length decreases in conveying direction and whose shellconsists of a perforated plate through which the solvent-adhesivedispersion can pass. The decrease in volume per unit length of theconveyor screw may be realized either by a variation in pitch of thescrew shaft or by a casing configuration which converges in conveyingdirection.

As an alternative, the separation of the dispersed adhesive from thechopped sheet material may be effected by introducing shearing forces ina vessel at the bottom of which two coaxial wing rotors are arrangedwhich rotate in opposite sense in a radial plane and between the winggroups of which there is a cylindrical separating zone. Particularlyhigh shearing forces are created in this cylindrical separating zone towhich the chopped sheet material is fed by flow created in the vessel,and these shear forces permit clear separation of the dispersed adhesivefrom the chopped sheet material, as with the first alternative.

The type of solvent used essentially depends on the type of adhesivewhich adheres to the chopped sheet material. Experience made in thetesting phase of the instant process has shown that the solventpreferably should have a boiling point of from 40° to 90° C., especiallyfrom 50° to 70° C.

The preferred solvent is benzine having a maximum benzene content of 1%.The limitation of the benzene content is made in view of safetyregulations. The benzine is light benzine consisting of straight chainor branched, preferably branched, saturated hydrocarbons, pentane andhexane being suitable in consideration of the preferred boiling points.However, as hexane is not preferred for use because of the riskinvolved, it is more advantageous to apply a mixture of pentane andhexane or, most preferably, pentane alone.

The preferred process temperature lies at a value between 25° and 35°C., and the dispersing time in the first stage is about 3 minutes.

Preferably the dispersion to be withdrawn from the first stage of theprocess and to be returned to the manufacturer of the sheet material hasa mixing ratio of approximately 20% of adhesive and 80% of solvent. Inthis manner the requirements both of the manufacturer of sheet materialand of the instant can be taken into account.

DESCRIPTION OF THE DRAWINGS

The drawing consists of a diagrammatic flow chart of the process stagesof a preferred embodiment of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the method, comprising a four-stage kneader ineach stage of the process will be explained below with reference to adiagrammatic flow chart.

The diagram illustrates a countercurrent method comprising altogetherthree stages. In a first stage 7 of the process, chopped sheet material4 coated with adhesive is supplied through a reservoir 2 to a firststage 10 of a kneading apparatus 16 including a total of four stages.This chopped sheet material 4 in practice may be wetted with water,whereby suppliers (manufacturers of sheet material) intend to preventthe chopped sheet material from agglomerating. The water thus enteringthe process does not disturb the process since it forms an azeotropetogether with the benzine and, during later reuse of the adhesive,evaporates with the solvent.

The solvent bath 1 within the kneading apparatus 16 is fed by theadhesive-solvent dispersion exiting from the conveyor screw 3 of thesecond stage 8 of the process. All the kneading stages 10 to 13 of thekneading apparatus 16 comprise respective kneading rotors (not shown)which pass along the chopped sheet material 4 in conveying direction (tothe left in the diagram) from one kneading stage to the next. After aresidence time (dispersing time) of the chopped sheet material of about3 minutes in the solvent bath 1, a pasty, honey-like dispersion hasformed of the adhesive and the solvent, this dispersion still adheres tothe chopped sheet material 4 when the latter issues from the fourthkneading stage 13. The kneading apparatus 16 is followed in conveyingdirection by a conveyor screw 3 having a conveyor shaft 6 whose volumeper unit length decreases in conveying direction due to convergence ofthe shell of the conveyor screw 3. This decrease in volume per unitlength causes squeezing forces to be introduced between the screw shaft6 and the inside of the shell of the conveyor screw 3, wherebymechanical separation of the dispersion adhering to the chopped sheetmaterial 4 is caused. This adhesive-solvent dispersion exits through aperforated plate 5 located in the shell of the conveyor screw 3. In thecase of this first stage 7 of the process, the exiting dispersionalready is the mixture which is returned to the manufacturer of thesheet material. It has a mixing ratio, for example, of 20:80.

The chopped sheet material 4 extruding from the conveyor screw 3 of thefirst stage 7 of the process already is largely free of adhesive and issupplied through a buffer reservoir 14 to a second stage 8 of theprocess. In this second stage 8 of the process the chopped sheetmaterial 4 is further cleaned and final separation of the adhesive fromthe chopped sheet material takes place. The second stage 8 of theprocess and the third stage 9 as well are of the same basic structure asthe first stage 7 of the process. For reasons of clarity, therefore, thesame members were not marked by reference numerals. The dispersionexiting through the perforated plate 5 of the conveyor screw 3 of thesecond stage 8 of the process is fed in countercurrent to the solventbath 1 of the first stage 7 of the process. The solvent bath of thesecond stage 8, on the other hand, receives its solvent (dispersion)from the conveyor screw 3 of the third stage 9 of the process whosesolvent bath is fed with fresh solvent through buffer reservoir 15. Thechopped sheet material 4 extruding from the conveyor screw 3 of thesecond stage 8 of the process likewise is supplied to the bufferreservoir 15 of the third process stage 9.

The countercurrent method thus resides in supplying the chopped sheetmaterial successively to the various process stages 7 to 9, while thefresh solvent is added in the third process stage 9 and then fed inopposite sequence, as an adhesive-solvent dispersion, first to thesecond stage 8 and then to the first stage 7 of the process.

In the process described, benzine is the preferred solvent used. It hasa boiling point between 50° and 70° C. and a maximum benzene content of1%. Chemically, the benzine preferably is pentane. Likewise conceivableis hexane or a mixture of pentane and hexane.

The process temperature preferably is maintained at a value between 25°and 35° C. at which a dispersing time in the first stage 7 of theprocess of about 3 minutes is obtainable.

What is claimed is:
 1. A method, of recycling plastic sheet materialcoated with adhesive and available in chopped form by separating theplastic from the adhesive, comprising the following steps:a) moving thechopped sheet material for a predetermined period of time in a solventfor the adhesive through a multi-stage kneader, so that the adhesivewill become dispersed in the solvent; b) subsequently separating theadhesive dispersed in the solvent from the chopped sheet material bymechanical force.
 2. A method, of recycling plastic sheet materialcoated with adhesive and available in chopped form by separating theplastic from the adhesive, comprising the following steps:a) moving thechopped sheet material for a predetermined period of time in a solventfor the adhesive under the impact of a mechanical force in a firstprocessing stage, so that the adhesive will become dispersed in thesolvent; b) separating the adhesive dispersed in the solvent from thechopped sheet material by mechanical force in such first processingstage; c) feeding the separated chopped sheet material from the firstprocessing stage to a second processing stage; d) feeding fresh solventto the separated chopped sheet material in the second processing stage;e) moving the chopped sheet material for a predetermined period of timein the solvent for the adhesive under the impact of mechanical force inthe second processing stage, so that the remaining adhesive will becomedispersed in the solvent; f) separating the solvent-adhesive dispersionfrom the chopped sheet material by mechanical force in the secondprocessing stage; and g) feeding the separated solvent-adhesivedispersion from the second processing stage to the first processingstage.
 3. A method, as set forth in claim 2, further including:h)feeding the separated chopped sheet material from the second processingstage to a third processing stage; i) feeding fresh solvent to theseparated chopped sheet material in the third processing stage; j)moving the chopped sheet material for a predetermined period of time inthe solvent for the adhesive under the impact of mechanical force in thethird processing stage, so that the remaining adhesive will becomedispersed in the solvent; k) separating the solvent-adhesive dispersionfrom the chopped sheet material by mechanical force in the thirdprocessing stage; and l) feeding the solvent-adhesive dispersion fromthe third processing stage to the second processing stage.
 4. A method,as set forth in claim 2 or 3, wherein the step of moving the choppedsheet material in a solvent under the impact of mechanical force in eachprocessing stage further comprises moving the chopped sheet materialthrough a multi-stage kneader.
 5. A method, as set forth in claim 1, 2or 3, wherein the step of separating the solvent-adhesive dispersionfrom the chopped sheet material in each processing stage furthercomprises applying squeezing forces to the chopped sheet material by ascrew conveyor.
 6. A method, as set forth in claim 1, 2 or 3, whereinthe step of separating the solvent-adhesive dispersion from the choppedsheet material in each processing stage further comprised applyingshearing forces in a screw conveyor.
 7. A method, as set forth in claim1, 2 or 3, wherein the solvent has a boiling point of from 40° C. to 90°C.
 8. A method, set forth in claim 7, wherein the solvent used isbenzine having a maximum benzine content of 1%.
 9. A method, as setforth in claim 1, 2 or 3, further including maintaining a processtemperature at a value between 25° C. and 35° C.
 10. A method as setforth in claim 1, 2 or 3, wherein the predetermined time of moving thechopped sheet material in the solvent in the first processing stage isabout 3 minutes.
 11. A method as claimed in claim 1, 2 or 3, whereinsolvent is supplied such that the solvent-adhesive dispersion separatedwill have a mixing ratio of approximately 20% of adhesive and 80% ofsolvent.